FIG. 1 shows the structure of a conventional satellite signal transmission apparatus. As shown, the apparatus includes two receiving antennas 1′ for receiving vertical and horizontal polarized signals sent from a satellite. The received vertical and horizontal signals are respectively amplified and down-converted at amplifiers 11′ and 12′ that together form an RF amplifying system A. The amplified and down-converted signals are then separately sent to two band-pass filters 13′ for filtering, so as to generate signals with fixed bandwidth. The filtered signals are then separately sent to mixers 2a′ and 2b′. 
The mixers 2a′ and 2b′ for the vertical and horizontal signals, respectively, receive oscillating frequencies of two different local oscillators 3′ and 4′, and separately mix the received oscillating frequencies with the above-mentioned signals having fixed bandwidth to generate two different mid-bands, which are filtered at a low-pass filter 5′ and a high-pass filter 6′, respectively, to remove noises therefrom.
The filtered signals are then sent to a multiplex adder 7′ to execute an addition operation. The resultant signals are then transmitted via a cable 8′.
In the above-described structure, the oscillating frequencies of the two local oscillators 3′, 4′ isolate the vertical and the horizontal signal from each other to generate two non-repeated mid-bands. However, an intermodulation interference would occur if the oscillating frequencies were not properly selected.
To enable better understanding of many drawbacks of the conventional satellite signal transmission apparatus of FIG. 1, an example thereof is now described in more details. Please refer to FIG. 1. A vertical receiving antenna 1′ receives a satellite vertical polarized signal having a frequency within the range from 3.7 to 4.2 GHz, and another horizontal receiving antenna 1′ receives a satellite horizontal polarized signal also having a frequency within the range from 3.7 to 4.2 GHz. The received vertical and horizontal signals are amplified and down-converted at the RF amplifying system A and then separately sent to the band-pass filters 13′ to generate two signals with fixed bandwidth, which are then sent to the mixers 2a and 2b, respectively. The vertical local oscillator 3′ has an oscillating frequency of 5.15 GHz, and the horizontal local oscillator 4′ has an oscillating frequency of 5.75 GHz. The oscillating frequencies of the two local oscillators 3′ and 4′ are output to the mixers 2a′ and 2b′, respectively, to mix with the frequencies from 3.7 to 4.2 GHz of the vertical and the horizontal signals with fixed bandwidth. A frequency subtraction operation is executed at the mixers 2a′, 2b′ to obtain a maximum vertical frequency of 1450 MHz (i.e., 5.15−3.7 GHz=1450 MHz) and a minimum vertical frequency of 950 MHz (i.e., 5.15−4.2 GHz=950 MHz), so as to generate the mid-band as shown in FIG. 2A. Meanwhile, a maximum horizontal frequency of 2050 MHz (i.e., 5.75−3.7 GHz=2050 MHz) and a minimum horizontal frequency of 1550 MHz (i.e., 5.75−4.2 GHz=1550 MHz) are similarly obtained to generate the other mid-band as shown in FIG. 2B. The resultant mid-bands are filtered at the low-pass and the high-pass filter 5′ and 6′, respectively, to remove noises therefrom. The filtered mid-bands are then sent to the multiplex adder 7′ to execute the addition operation to generate the combined vertical and horizontal mid-bands, as shown in FIG. 2C.
A second harmonic difference between the oscillating frequencies of the vertical and the horizontal local oscillator 3′, 4′ is (2×5.75)GHz−(2×5.15)GHz=1200 MHz and a third harmonic difference between the oscillating frequencies of the vertical and the horizontal local oscillator 3′, 4′ is (3×5.75)GHz−(3×5.15)GHz=1800 MHz. The above two harmonic differences of 1200 MHz and 1800 MHz fall in band ranges from 950 to 1450 MHz and from 1500 to 2050 MHz, respectively, as shown in FIG. 2D, and therefore result in the intermodulation interference and the problem of failed reception of video signals.